THz radiation occupies a large portion of the electromagnetic spectrum between the infrared and microwave bands, namely the frequency interval from 0.1 to 10 THz, and is a developing frontier in imaging science and technology. In contrast to the relatively well-developed techniques for imaging at microwave and optical frequencies, however, there has been only limited basic research, new initiatives and advanced technology developments in the THz band.
THz time-domain spectroscopy (THz TDS) allows exploration of the rich spectroscopic information on molecular vibrations, rotations, and other low-energy transitions in biological and organic compounds, and semiconductor structures. Biological and organic compounds have distinct signatures within the THz region of the electromagnetic spectrum, such as molecular vibrational and rotational levels, and their chemical compositions can be examined with THz wave microscopic systems.
Unlike X-rays, THz radiation has low-photon energy (4 meV @ 1 THz), low average power (nW to μW) and does not subject biological tissue to harmful radiation. THz radiation can be focused to give sharper images. In addition, THz radiation provides spectroscopic information about the chemical composition, as well as the shape and location of the samples it is imaging.
Unlike common optical spectroscopes, which only measure the intensity of light at specific frequencies, THz time-domain spectroscopic techniques directly measure the THz wave's temporal electric field. Fourier transformation of this time-domain data gives the amplitude and phase of the THz wave pulse, therefore providing the real and imaginary parts of the dielectric constant without the use of Kramers-Kronig relations. This allows precise measurements of the refractive index and absorption coefficient of samples that interact with the THz waves.